Abstract:

The effectiveness of the Tri-isotropic (TRISO) fuel particle to perform as an effective barrier to fission product migration and retain all fission products within the fuel particle is based primarily on the ability of the silicon carbide (SiC) layer to maintain its integrity in spite of the operating conditions of the Pebble Bed Modular Reactor (PBMR). However, because of the complexity of the fission product migration phenomena, fission products continue to escape through the fuel barriers in spite all efforts having being done to confine them within the fuel kernel. This suggests that there is still a lot of work still to be done to understand fission gas migration phenomena, which is the subject of this Research Report. This Research Report will concentrate on the analysis of the gaseous fission products released during the HFR-K5 fuel test, as part of the validation of the PBMR gaseous fission product release calculation model. Investigations done on similar fuel by other researchers suggests that some of the most important factors that determine the rate of fission product migration are irradiation temperature, neutron fluence, the diffusion mechanism i.e. vacancy as opposed to interstitial mechanism and the duration of irradiations. The variation of fission product release as a function of these variables has been investigated extensively in the present study. In addition to investigating the correlation between release and these factors, radiation effects such as dimensional change has also been analysed as volumetric change of the TRISO coated particle and is an important parameter in reactor design. The design of the defueling chute of the PBMR must take this into consideration as the diameter of the fuel pebble will have swollen as a result of radiation damage. Based on the findings of the present research study the recommendations of how the research findings can be implemented to provide an improved PBMR fuel are made.